The present invention relates to a power source apparatus for supplying power to a grid or shield of a charger.
In order to eliminate irregular charging of and charge variations in a charger in a printer or a copying machine which utilizes an electrophotographic process and to stabilize a surface potential of a photosensitive drum, a grid or shield wire is arranged near the photosensitive drum and applied with a stable voltage. A conventional power source system uses a power source for generating the stable voltage by boosting a voltage by a DC-DC converter.
The DC-DC converter system has been popular since a voltage of a wide range, e.g., several tens of volts to about 1 kV, can be generated by a transistor of a low withstand voltage and the output voltage can be easily stabilized.
A reverse current with respect to a polarity of an output from the DC-DC converter generally flows to degrade power efficiency. As a result, a converter transformer becomes bulky and expensive, and its temperature rise is undesirably increased.
A conventional arrangement of a means for applying a bias voltage to a grid of a charger will be described with reference to FIG. 10.
FIG. 10 is a schematic circuit diagram showing a conventional means for applying a bias voltage to a grid of a charger in an image forming apparatus.
A charger 2 is located near the outer surface of a photosensitive drum 1, and a high-voltage power source 3 is connected to a charge wire 7 to apply a high voltage of -6 kV thereto. One input terminal of an error amplifier 4 is connected to a connecting point of series-connected resistors R.sub.1 and R.sub.2, and the other input terminal serves as a reference signal input terminal P.sub.1. The output terminal of the error amplifier 4 is connected to a PWM (Pulse Width Modulation) circuit 5. A grid 6 is arranged near the outer surface of the photosensitive drum 1 at an opening of the charger 2 and is applied with a voltage in the range of -100 V to -900 V. The grid 6 is connected to one end of the series circuit of the resistors R.sub.1 and R.sub.2 and to the secondary side of a converter transformer T.sub.1 through an output diode D.sub.1. The collector of a switching transistor Tr.sub.1 is connected to the primary side of the converter transformer T.sub. 1.
With the above arrangement shown in FIG. 10, the high voltage of -6 kV is applied from the high-voltage power source 3 to the charge wire 7 in the charger 2. A voltage in the range of -100 V to -900 V must be normally applied to the grid 6. A current flowing through the charge wire 7 is partially supplied to the grid 6. When a voltage of -100 V is applied to the grid 6, a current of 200 .mu.A flows in a direction indicated by an arrow 6a. When a voltage of -900 V is applied to the grid 6, a current of 30 .mu.A flows in the direction of the arrow 6a. The polarity of this current is set to cut off the output diode D.sub.1, and the current must be consumed through the resistors R.sub.1 and R.sub.2 When the voltage of -100 V is applied to the grid 6, a series resistance of the resistors R.sub.1 and R.sub.2 must be 500 k.OMEGA. or less to reduce the current of 200 .mu.A. When the voltage of -900 V is applied to the grid 6, a bleeder current of 1.8 mA flows through the resistors R.sub.1 and R.sub.2, so that their power consumption is 1.62 W. A required power is 20 mW to 30 mW, thus resulting in extreme power loss.
As can be apparent from the above description, in the conventional power source system, the sizes of the resistors R.sub.1 and R.sub.2 and the converter transformer T.sub.1 are increased, and a high-power transistor must be used as the switching transistor Tr.sub.1 accordingly, thus generating a large amount of heat.